Field of the Invention
The invention relates to a liquid crystal cell--also referred to below as display cell--having a ferroelectric, chiral smectic liquid crystal layer, namely a display cell according to the preamble of claim 1.
Known liquid crystal cells have a nematic liquid crystal configuration and, in accordance with this configuration, are referred to as TN or STN cells. Here, TN is Twisted Nematic and STN is Super Twisted Nematic. Such cells are adequate for many electro-optical applications. Thus, most of the liquid crystal displays known today have active matrix-driven TN cells. In these display cells, a display segment which can be electrically driven by means of an associated semiconductor circuit is coordinated with each image point. Many of the electro-optical requirements, such as contrast, gray steps and orientability of the liquid crystal layer, are optimally met by TN displays, which explains their wide use.
However, the TN display cells cannot meet all requirements to an equal extent. Important disadvantages are a limited angular range of view and long switching times. These disadvantages are due to the physics of the TN cell and cannot be easily overcome. Thus, in this display cell, switching on is electrically driven but not switching off. The result of this is that the switching times, i.e. the speeds for image build-up and image clearance, determined by the viscosity and the elastic restoring forces of the nematic liquid crystal mixture, are indeed limited. TN cells are therefore not intended for displaying sequences of images following one another in quick succession. This applies, for example, to many modern multimedia applications and the like.
Attempts were made at an early stage to find an alternative to the TN display cell. Possible faster media are the ferroelectric, chiral smectic liquid crystals. These have a spontaneous polarization which permits much stronger coupling of the liquid crystal to the electric field than is possible in the case of TN cells. In particular, however, this coupling is linear in the field. This has two important consequences. First, the torques are large even at low driving voltages and, secondly, in a display cell containing a ferroelectric, chiral smectic liquid crystal layer, both switching on and switching off are electrically driven and thus relatively rapid.
Ferroelectric, chiral smectic liquid crystal cells have a birefringent liquid crystal mixture which is also referred to below as an S.sub.c * layer (here, * represents the chirality of the liquid crystal layer), optionally forms a helical configuration and can be influenced or deformed by the action of an electric field so that its optical anisotropy changes.
The term smectic denotes a layered structure which is based on the fact that the molecules of the liquid crystal mixture have polarizable cores and apolar side chains. In the smectic phase, the polar cores in this case are arranged in the smectic layers, which in turn are separated from one another by apolar layers comprising side chains. In ferroelectric liquid crystal cells, the smectic layers are essentially perpendicular to the plates of the display cell.
The S.sub.c * layer is also distinguished by further properties. Thus, the molecular cores belonging to a smectic layer and arranged essentially parallel to one another are not perpendicular to the associated planes of the layers but are tilted away from the normal of the plane by an angle .theta.. Further important properties of the S.sub.c * layer are based on their chirality. In fact, this gives the S.sub.c * layer a spontaneous polarization P.sub.s, whose direction lies in the plane of the smectic layers and perpendicular to the molecules. This means that an electric field applied to the display cell interacts strongly with this spontaneous polarization, permitting a substantial reduction in the switching times known for TN and STN cells. The chirality furthermore results in the axes of the liquid crystal molecules being rotated relative to one another from layer to layer without external forces, with the result that a helix with the pitch p forms.